Null Object

null.cpp

#include <iostream>

class ChaseStrategy{
public:
    virtual void chase() = 0;
};

class RandomChase : public ChaseStrategy {
    void chase() override {
        std::cout << "Randomly wander around\n";
    }
};

class DirectedChase : public ChaseStrategy {
    void chase() override {
        std::cout << "If I see target, move towards it\n";
    }
};

class NullChase : public ChaseStrategy {
    void chase() override {
       // do nothing
    }
};

NullChase nullChase;

class Ghost {
    ChaseStrategy* chaseStrategy;
public:
    Ghost():chaseStrategy(&nullChase) {}

    void set(ChaseStrategy* newstrategy) {
        chaseStrategy = newstrategy;
    }

    void doIt() { 
        chaseStrategy->chase(); //safe because of null object 
    }
};

int main()
{
    Ghost pinky;
    pinky.doIt(); 

    pinky.set(new DirectedChase);
    pinky.doIt();

}

withoutnull.cpp

#include <iostream>

class ChaseStrategy{
public:
    virtual void chase() = 0;
};

class RandomChase : public ChaseStrategy {
    void chase() override {
        std::cout << "Randomly wander around\n";
    }
};

class DirectedChase : public ChaseStrategy {
    void chase() override {
        std::cout << "If I see target, move towards it\n";
    }
};


class Ghost {
    ChaseStrategy* chaseStrategy;
public:
    Ghost() {}

    void set(ChaseStrategy* newstrategy) {
         chaseStrategy = newstrategy;
    }

    void doIt() { 
        if(chaseStrategy != null) // better check!
            chaseStrategy->chase(); 
    }
};

int main()
{
    Ghost pinky;
    pinky.doIt(); // no strategy yet, be carefull!

    pinky.set(new DirectedChase);
    pinky.doIt();

}